1. Field of the Invention
The present invention relates to a method of making highly sensitive photoconductive particles utilizable as a photoconductive toner for one-shot color electrophotographic image formation.
2. Description of the Prior Art
In an electrophotographic color copying machine, it is generally well known that toner particles are dyed in three different primary colors based on the subtractive principle, cyan, magenta and yellow. In order to produce color image reproduction, the color copying machine now available on the market is so designed as to perform a process of exposure, development and fixing three times, one for each primary color.
Since the repetition of the process of exposure, development and fixing for each primary color requires a fairly long time to complete reproduction of an image of one particular color by the use of an electrophotographic technique, one-shot color electrophotographic image formation has been proposed wherein a color image reproduction can be accomplished by a single process of exposure, development and fixing. In the practice of this one-shot color electrophotographic image formation method, photoconductive toner particles such as disclosed in, for example, the Japanese Unexamined Patent Publications No. 59-113448 and No. 60-31149 laid open to public inspection on June 30, 1984, and Feb. 10, 1985, respectively, are employed.
According to these prior publications, the photoconductive toner is made up of powdery photoconductive particles as a principal component, pigments of three different primary colors based on the subtractive principle, charge controller and binder. The principal component, that is, the photoconductive powder, is employed in the form of photoconductive material such as, for example, zinc oxide (ZnO) mixed with three types of spectral sensitizers such as pigments.
The photoconductive toner according to any one of these prior publications exhibits a problem in that, since the photoconductive material is merely mixed with the three types of spectral sensitizers, both are susceptible to separation and the photoconductive toner can not sustain a spectral sensitization for a prolonged period of time. Therefore, a binder made of synthetic resin is currently employed to bond the photoconductive toner particles with the three types of spectral sensitizers to minimize the separation.
On the other hand, the paper presented by H. Watanabe et al. in "JAPAN DISPLAY '86" and entitled "P3.3 Photoconductive Particles For One-shot Color Electrophotographic Image Formation", pages 568-571 which paper was published in 1986, discloses a similar photoconductive toner utilizing ZnO powder with pigments selectively responsive to the three different primary colors based on the additive principle, blue, green and red. According to this paper, in order to avoid the possible separation is discussed hereinbefore, the ZnO particles are allowed to adsorb the pigments selectively responsive to the three primary colors of the additive principle and then are dispersed in polymer to produce the photoconductive toner. The mere adsorption of the pigments by the ZnO powder does not eliminate the possibility of separation of the pigments from the powder particles.
In any event, the prior art methods of making the photoconductive toner are such that particles of metal oxide such as, for example, TiO.sub.2 or ZnO are mixed, adsorbed, or bonded with the use of a resinous binder, with different pigments having selectivities of response to the three primary colors of the subtractive principle, thereby to produce the photoconductive particles for use in an electrophotographic color copying machine for the electrophotographic formation of color images.
The photoconductive toner made by any one of the prior art methods has a problem in that, since the force of contact between the metal oxide particles and the pigments is small with no energy band continued at the interface therebetween, the spectral sensitization of the pigments cannot be smoothly accomplished. Moreover, since the spectral sensitizing characteristics of the pigments are not satisfactory as discussed above, the spectral sensitivity to the three primary colors of the additive principle is very low with their spectral sensitivity curves greatly overlapping with each other as compared with their peak values.
In view of the foregoing problems, when the photoconductive toner produced according to the prior art methods is used in a color copying machine for the electrophotographic formation of the color image, the color rendering capability is not satisfactory and exhibits insufficient hue, sometimes accompanied by an irregular color reproduction and sometimes by color smear. In other words, with the conventional photoconductive toner now available on the market, distinct color reproduction cannot be achieved.